There are known various types of such a glow plug with a combustion pressure sensor (hereinafter, may be referred to merely as a glow plug) (refer to, for example, Patent Document 1). FIG. 10 is a longitudinal, sectional view showing a simplified forward portion of a glow plug 100 of the same type. The glow plug 100 of FIG. 10 is configured such that a rodlike (circular columnar) ceramic heater (hereinafter, may be referred to as a heater) 200 having, at its forward end 201, a forward heat-generating element 205 formed of a resistance heat-generating element and adapted to generate heat through application of electricity is disposed in a tubular housing 110 with the forward end 201 protruding forward from a forward end 136 of the housing 110. In the present application, a forward end of a glow plug or forward ends of its components (members) correspond to their lower ends in FIG. 10, and rear ends correspond to their opposite ends (upper ends). The “broken line” in the enlarged view of FIG. 10 indicates a portion, in the vicinity of a combustion chamber, of a plug hole of an engine head to which the glow plug 100 is mounted. In the glow plug 100, the heater 200 is disposed in the housing 110 in such a manner as to be displaceable by a very small distance along an axis G (axial direction) in response to received combustion pressure, whereby a sensor senses the pressure. In the glow plug of FIG. 10, the heater 200 is disposed with a space (annular gap) formed in a radial direction in cooperation with the inner circumferential surface of the housing 110; when the heater 200 is pressed rearward by combustion pressure and is thus displaced rearward in relation to the housing 110, the displacement (or associated pressure) is transmitted to a sensor 400 incorporated in the housing 110, through a pressure transmission member 350 formed of a shaft member (e.g., a hollow shaft member) fixed to the heater 200; and the sensor outputs an electric signal corresponding to relative displacement of the heater 200 or associated pressure.
The glow plug 100 has a very small space in a radial direction between the inner circumferential surface of a forward end portion of the housing 110 and the outer circumferential surface of the heater 200, and the space assumes the form of an annular space (gap) communicating with an opening at the forward end 136. Thus, in order to prevent the problem that combustion gas having high temperature and high pressure enters the space K, reaches rearward within the housing 110, and leaks out to the ambient atmosphere, a displacement member (membrane) 300 formed of a heat-resistant material is provided. The displacement member 300 has the form of, for example, a ring or a short tube, is disposed around the heater 200, and is circumferentially welded to the housing 110 side and to the heater 200 side, thereby axially blocking the space K in a sealed condition. Meanwhile, since the displacement member 300 needs to allow displacement of the heater 200 along the axis G, the displacement member 300 is formed of a thin metal sheet (membrane) so as to be deformable. In the glow plug 100 of FIG. 10, the inner circumferential surface of a forward end portion of the housing 110 is expanded in diameter at a position located axially inward of the opening of the forward end 136 of the housing 110; the displacement member 300 has the form of a diameter-differed short cylinder such that a forward portion has a small diameter, while a rear portion has a large diameter; and the large-diameter portion of the displacement member 300 is disposed in the diameter-expanded space K located at an axially deep position and is joined to the housing 110 side, while the small-diameter portion of the displacement member 300 is joined to the heater 200 (outer circumferential surface) side. An intermediate portion of the thus-configured displacement member 300 is deformed forward or rearward as is a diaphragm, in response to displacement along the axis G of the heater 200 in relation to the housing 110. Notably, since the displacement member 300 made of metal cannot be directly welded to the ceramic heater 200, according to ordinary practice, the displacement member 300 is welded to a metal pipe 250 which is externally fitted and fixed to the heater 200.